A primary value in video recording for surveillance and security derives from gathering an accurate record of image information from cameras and maintaining that record for some duration of time, or archive interval (archive intervals of 24 hours to two months are common). Conventional single-camera and multi-camera installations almost universally use videotape recorders (VTR), primarily the VHS format of videocassette recorders (VCR). to store the large volume of camera images needed to record and replay motion activity over useful archive intervals.
Operation and maintenance of VCRs presents a considerable cost and workload overhead in security systems, especially since most security systems are operated continuously, day in and day out, for the life of the installation. VCRs generally require overhaul for every 10,000 hours to 20,000 hours of operation (approximately 1-2 years of continuous operation), and pictures can degrade as the VCR accumulates hours of operation toward an overhaul date.
The relatively small number of images (432,000) that can be stored on a VHS cassette dictates that operators make frequent cassette changes on an ongoing basis, or employ many VCRs recording at reduced rates so that cassette changes can be made according to a convenient schedule (e.g. once every 24 hours). Long term unattended recording by repeated cycling of a single tape is not advised since tape life is on the order of 25 cycles, and no warning of image degradation due to tape wear is provided on standard machines.
In short, VCRs are bulky relative to the number of images stored and are prone to mechanical wear which causes picture degradation. VCRs require maintenance far more frequently than any other element of the surveillance system. It is acknowledged in the security industry that the VCR is the weak link in the task of recording massive amounts of image information on a routine year-in and year-out basis.
Proposals for consumer VCRs and camcorders based on digital rather than analog techniques are nearing acceptance at the time of writing. A proposed Standard Definition (SD) format for digital recording stores four and a half hours of NTSC equivalent video on a cassette that occupies 30% the volume of a VHS cassette. This represents a nearly eight-fold improvement in picture density over VHS technology. The mechanics of digital VCR recording, however, have been chosen to link head rotational speed and tape speed to frame rate in order to easily implement features such as editing, still playback, and multi-speed playback. This requires the compressed data stream to be of a fixed average length over intervals such as a frame or a fraction of a frame. This constraint limits the compression ratios that could be attained were a highly-variable-ratio compression scheme otherwise used. To achieve greater picture density, an alternate recording format will be required.
Many techniques have been devised to accelerate the rate of image recording in response to trigger events such as door switch closure or motion detected by infrared or video motion detectors. The objective is to raise the probability of capturing significant information at the time of an event. Many security managers elect not to use such techniques since they disrupt the uniformity of media fill times and the scheduling for changing recording media. The same problem of non-uniform media fill rates is brought on by a similar technique of recording extra images occurring prior to the trigger event; the so-called `pre-trigger` recording techniques. Pre-trigger recording, which has become practical with the advent of digital video methods, could be utilized more widely were the problem of media fill variability solved.
VHS videocassette recorders deliver horizontal resolution for color Images of approximately 2401 TV lines for standard models and approximately 400 TV lines for S-VHS models. Many videocameras available for security have horizontal resolutions of 330 or 460 TV lines.
Performance of either the camera or the VCR becomes wasted in the mismatch. Also, since resolution of VCRs is fixed by design, techniques to raise or enhance the recorded image resolution in response to particular monitoring conditions are not practiced in the existing art of industrial surveillance.
Several techniques have evolved in the field of security and surveillance for adding other types of data into the video stream for recording on the VCR. For example, in U.S. Pat. No. 5,216,502 Katz describes how to record transaction data from cashier lanes with the video images from the lanes by impressing the digital transaction data onto the audio track of a VCR, or onto the end lines of the video frame recorded on the VCR. In U.S. Pat. No. 4,949,173 Mitsuhashi discusses methods to carry continuous-time audio along with intermittently recorded (time-lapse) video images. Multiplexer devices (discussed further below) generally code alarm status information and camera source identification onto the video input prior to storage on the VCR.
Multiplexer devices are commonly used in multi-camera installations to choose single fields or frames from any of the asynchronous camera sources to create a video image sequence for recording onto a VCR. Multiplexers are employed to maximize the rate of coverage at each camera view (to minimize the time between images) while minimizing the total number of images that must be stored on the recorder. Multiplexers also provide time-base correction for continuous or time-lapse recording VCRs, which is necessary since the rotating head-drum in a VCR cannot instantly synchronize to a randomly arriving field. Commercial multiplexers for surveillance employ video analog-to-digital-to-analog techniques and constitute a significant fraction of overall system cost.
FIG. 1 shows a representative conventional multi-camera system using a multiplexer 60. The time-multiplexed output of the multiplexer is applied to a first VCR 62 for recording. Playback of previously recorded information from a second VCR 64 is returned to a time-multiplexed input on the multiplexer. During playback the multiplexer selects one camera view from the time-multiplexed playback signal and displays this camera view on a monitor 66. The multiplexer can also deliver live camera images to the monitor.